This invention relates to the medical field of orthopaedics and joint replacement, in particular. Modular artificial joints have several components that must be assembled and placed in the patient to reconstruct a joint. While modular joints provide the ability to custom fit an artificial joint to a patient""s anatomy, the connection between the components must be without relative movement after implantation. This invention is directed to a modular artificial joint construction and method which provide a locking mechanism to secure the components immovably together and a visual indication that the components are properly assembled.
In replacing a hip joint, the head of the femur is removed along with the ball. The trochanter portion of the femur is shaped and prepared for receiving the prosthesis so that the artificial joint will closely approximate the natural hip.
Earlier artificial hip joints were made of one-piece construction requiring a large inventory of prosthesis to accommodate the various sized patients. The modular artificial joint has two or three or more elements which replace the natural hip. By manufacturing these components with interchangeable connections but different external sizes, inventories may be smaller because of the ability to mix and match components. Also, the modular prosthesis provides more flexibility in customizing the various components of a joint to the various parts of a patient""s natural joint.
In a three piece artificial hip joint, the various sized components of the joint that may be selected are the intramedullary rod, the trochanter and the neck. The intramedullary rod is inserted into the end of the femur. The rod acts as a stabilizer in maintaining the artificial joint in the axis of the femur. The upper portion of the rod which extends toward the proximal end of the femur is fitted into a trochanter element which is shaped like the removed broad head of the femur which it replaces. This element, along with the rod, is used to adjust the length of the prosthesis to approximate the natural length of the femur.
The natural trochanter is the broadened area offset from the end of the femur. The natural trochanter may be at any radial angle about the axis of the femur. This natural angular relationship must be reproduced by the intramedullary rod and the artificial trochanter. The artificial trochanter is seated on the end of the patient""s femur and is the main load bearing element of the prosthesis. It is important that this load, which is mostly compression, is transmitted along the axis of the femur.
A neck element is inserted into the trochanter element and carries an extension onto which the ball joint will be fixed. The horizontal angle between the trochanter and the neck extension is variable to reproduce the anteversion angle of the patient""s natural joint. The neck carries cantilevered forces in torque and compression between the acetabulum and the trochanter. It is also important that these forces do not result in relative movement between the trochanter and the neck.
All these elements have a central bore and are permanently secured together by a compression screw which is inserted into the neck element, extends through the trochanter element, and is threaded into the upper end of the rod. In some cases, the intramedullary rod may be attached to the bone with bone cement while, in other cases the cement is omitted.
When the cement is omitted, the placement and fixation of the intramedullary rod becomes more critical to pain free usage of the prosthesis. Further, it is most important that the intramedullary rod not be disturbed after insertion since this would corrupt the union between the rod and the interior of the femur.
In order to maintain the original union between the femur and the intramedullary rod, modular prosthesis have been developed to allow rotational adjustment of the several parts or elements about the rod before the placement of the prosthesis to more closely reproduce the natural structure of the hip. The modular concept also allows the selection of different sized elements, before or during surgery, to more closely approximate the natural joint.
With the advantage of flexibility gained by modular prosthesis, there comes the requirement that there be no movement between the several parts or elements after implantation. These movements may cause misalignment of the joint resulting in increased pain, trauma to the joint and, even, dislocation of the joint.
In a modular hip prosthesis, the neck is subjected to cantilevered forces of torque and compression which are transmitted to the body of the prosthesis at the junction between the neck and body. If there is any angular movement between the neck and body at this junction, these forces are magnified and transmitted to the junction between the body and the bone thereby degrading the entire prosthesis. Prosthesis which have line seals or screw thread unions between these elements of the prosthesis are subject to relative movement between the elements. For example, when two cones are telescoped together the junction is a circular line seal about the circumference of the cones. This union can pivot about the line seal. Similarly, when a bolt or screw is used to tighten a connection between two pieces, the leading threads take most of the stress which is not evenly distributed along the shaft of the bolt.
The prior art is replete with artificial prosthesis and hip joints, in particular.
Illustrative of the state of the art are U.S. Pat. No. 5,725,592 to White et al, U.S. Pat. No. 5,876,459 and U.S. Pat. No. 5,506,644 to Powell which disclose modular hip joints having a stem, one end of which is inserted in the intramedullary canal. The other end of the stem is tapered to fit within a second, neck, element. The neck ultimately supports the ball joint. A sleeve element is placed over the junction of the first two-elements. All three elements are rotationally movable relative to each other. A bolt is driven through the bore of the neck and stem to fix the elements together.
In the Powell patents the bolt deforms a portion of the interconnected elements for a friction fit between the neck and the stem. These prior art patents disclose that the sleeve may have a polygonal shaped bore with the articulating elements having corresponding shaped portions. The interconnected elements of these hip joints do not form a static lock between each other but require a deformation of one or more elements before a friction fit is established. The deformation and friction fit is between the stem and the neck rather than the sleeve and the stem.
U.S. Pat. No. 5,653,765, to McTighe et al discloses a modular hip joint with a stem, an intermediate shoulder portion, and a proximal shoulder piece which attaches to the ball. The stem and the intermediate shoulder portion have interengaging teeth on the corresponding ends of each by which they are connected. This end-to-end connection allows for rotational movement of the elements relative to each other. The proximal shoulder piece and the intermediate shoulder piece also have an end-to-end toothed connection for rotational adjustment. This construction has two movable end-to-end connections which provide good flexibility for rotation of the elements but have small surface areas of fixation to each other limited to the surfaces of the interengaged teeth.
U.S. Pat. No. 4,851,007 to Gray discloses a modular hip prosthesis with a tapered fit between the intramedullary rod and the neck. In this prosthesis, there is no bolt used to fix the elements together.
In the instant invention a modular prosthesis is taught which has an intramedullary rod element which is to be inserted in a bone. The rod has a shaped proximal portion which is telescoped into one end of a bore in a body or trochanter element. The mating surfaces of the shaped rod and the body bore form a rotationally immovable connection. A neck element is telescoped into the other end of the body bore permitting rotational adjustment. All the elements are locked together by a compression screw through the neck and rod.
The method steps involve choosing the proper sized elements for a best-fit intra-operative prosthesis for the patient. The elements are combined together in an initial friction fit assembly in which the elements are relatively movable. This assembly leaves a slight gap between the neck and the body or trochanter. This gap can be visualized as a gauge of the final assembly of the prosthesis. When all the variables of the patient""s anatomy are replicated in the assembly, the prosthesis is subjected to a much greater longitudinal compression which closes the gap and fixes the elements immovably together. The compression screw is then inserted and torqued down to hold the elements in the assembled condition. The surgeon can visually check the prosthesis before implantation to verify that the assembly is complete and the elements are immovable with regard to each other.
The invention can be used as a prosthesis for various long bones in the body including, but not limited to, the femur, humerus, tibia, and fibula. The invention will be explained in relation to the femur.
In a particularly preferred embodiment of the instant invention a modular prosthesis is described for use as a hip replacement having an intramedullary rod, a trochanter and a neck. The intramedullary rod has a distal end adapted for insertion into the intramedullary canal of the femur and a proximal end. The proximal end has a reduced radius and a circumference with opposite planar surfaces joined by curved surfaces. The proximal end includes a screw threaded blind bore along the longitudinal axis of the intramedullary rod.
The trochanter or body has a narrow distal end and a larger proximal end forming an external shape approximating the natural bone. The artificial trochanter has a through bore from the distal end to the proximal end, with the proximal and distal ends of the through bore being tapered and having a smooth circumference. The intermediate portion of the through bore has a circumference with opposite planar sides joined by curved surfaces. The circumference of the trochanter bore and the circumference of the proximal end of the intramedullary rod telescope together with the opposite planar surfaces in intimate contact with each other forming a rotationally secure connection with the artificial trochanter approximating the position of the natural trochanter.
The neck has a conical extension about a through bore adapted to be inserted into the proximal end of the through bore of said trochanter. The proximal end of the through bore of the body or trochanter is tapered. The tapers of the conical extension and the through bore are complementary in size and shape. The through bore extension of the neck and the proximal end of the through bore in the trochanter telescope together forming an initially rotationally adjustable connection.
The proximal end of the through bore in the neck has an enlarged countersunk bore and the distal end of the through bore telescopes over the proximal end of the intramedullary rod. The initial assembly has a visually discernable gap between the distal end of the neck and the proximal end of the body or trochanter.
Upon completion of all adjustments necessary for the prosthesis to approximate the natural bone, final assembly is carried out with substantial longitudinal compression of the entire prosthesis sufficient to close the visual gap and force fit the complementary tapered walls of the neck and the body together in an area seal.
A threaded compression screw is disposed in the countersunk bore and threadably engaged with the screw threads in said proximal end of said intramedullary rod forming a locked integral prosthesis. The compression screw is turned until a recommended ft-lbs of torque is achieved.
Accordingly, it is an objective of the instant invention to provide an artificial joint with an intramedullary rod element which is connected with the body or trochanter element in such a manner as to prevent any relative movement between the elements. Rotational movement, in this context, refers to the turning of either element in a plane normal to the common longitudinal axis of the elements.
It is a further objective of the instant invention to provide the body or trochanter and the neck with a locking mechanism to rigidly secure the components together to prevent relative rotation.
It is another objective of this invention to provide a compression screw engaging the neck and the intramedullary rod through the intermediate body thereby securing the assembly with a particular torque value.
It is a still further objective of the invention provide a method of assembly of the elements to provide for adjustments peculiar to an individual and final assembly into a rigid prosthesis.
Other objectives and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.